1. Field of the Invention
The present invention relates to an optical switching element in which light transmittance can be controlled by using interference of the light.
2. Description of the Related Art
In recent years, liquid crystal elements have been used a means for effecting light modulation. For example, when a torsional nematic liquid crystal element is used, a pair of polarizing plates must be provided. However, in a system equipped with polarizing plates, because there is a large loss in the amount of transmitted light at the polarizing plates, light modulation must be effected due to this loss.
As a means for effecting light modulation, there is an optical switching element which varies the transmittance of the liquid crystal so as to set the intensity of the transmitted light to two values of brightness. For example, there are structures utilizing a complex refraction effect which is obtained by controlling the complex refraction of the layer of nematic-type liquid crystal. Layers adjacent to the liquid crystal have different refractive indices, and the refractive index of the layer of liquid crystal can be varied by the application of an electric field.
When such an optical switching element is used, for example, in a display device, the amount of light of the light source must be increased in consideration of the amount of light which is lost. Further, because the capacity of the light source is thereby increased, the amount of electric power consumed increases. An increase in consumption of electric power is not desirable because, for example, when the element is used in a portable computer, the operation time is shortened.
Moreover, when such an optical switching element is used as a shutter for exposure, either the amount of light of the light source must be increased or the exposure time must be lengthened to compensate for the amount of light which is lost. In particular, addressing this drawback by lengthening the exposure time may result in a loss of throughput at a printer or the like.
Further, in recent years, attempts have been made to use the above-described switching element in a display device to display highly detailed images on the display device. For example, when a television image is displayed, the number of scan lines necessary for one screen is 400 or more. However, in a matrix-type liquid crystal panel, the number of scan electrodes which can be disposed within the panel is limited to around 100. In order to address this matter, the signal electrodes are divided and the number of scan electrodes forming one screen is increased equivalently, and the plurality of scan electrodes which are connected commonly are scanned simultaneously.
As described above, in an optical system using polarizing plates, it is difficult to avoid the drawback of loss of the amount of light at the polarizing plates and to modulate light efficiently. Further, in structures utilizing the interference effect of a nematic liquid crystal, in order to achieve the desired interference effect, it is necessary to consider the refractive indices of a plurality of layers and it is difficult to design a structure in which the desired interference effect is obtained only at a target wavelength.
On the other hand, an example of a switching means using a nematic-type liquid crystal and effectively utilizing light is disclosed in Japanese Patent Application Laid-Open No. 4-140714. In this means, light incident on a complex refractive prism is separated into P polarized light and S polarized light. The S polarized light is deflected into P polarized light at a halfwave plate. The two sets of P polarized light are made incident on the nematic-type liquid crystal. In accordance with the above-described means, the efficiency with which light is used can be increased by using the polarized light of two directions. However, the loss of light at the polarization conversion system and the complexity of the structure provided with the polarization conversion system cannot be avoided.
Further, when highly detailed images are displayed, the signal electrodes within the panel are divided. As a result, it is necessary to lead a signal wire out from each of the divided signal electrodes. Because the signal wires are lead out from upper and lower portions of the signal electrodes, it is necessary to divide this divided structure into an upper structure and a lower structure. Further, the number of scan electrodes which are multiplex-driven in a simple matrix panel depends on the responsiveness of the liquid crystal. As a result, the number of scan electrodes is limited, and it is difficult to display a highly detailed image.